Information transfer over an optical fiber transmission system can be increased by multiplexing several signals on a single fiber. In wavelength-division-multiplexing ("WDM"), a plurality of optical sources are used to transmit a plurality of optical signals on a single optical fiber. Each source operates at a different wavelength.
In high capacity WDM systems, the lasers must operate at precisely determined wavelengths with fine wavelength separations from one laser to the next of about 0.8 to 1.6 nanometers (nm). Such precise wavelength control can be achieved by using a distributed feedback (DFB) laser.
DFB lasers employ a periodic structure, typically a corrugated slab or grating known as a distributed Bragg diffraction grating to achieve single frequency operation. The grating is designed so that light of a particular wavelength will be reflected constructively. This is known as Bragg reflection. Lasing occurs at this particular wavelength, the Bragg wavelength, with diffraction losses preventing other modes from lasing. It is understood that as used herein, "distributed feedback" or "DFB" lasers refer to distributed Bragg reflector (DBR) lasers, as well.
One way to alter the operating wavelength of a DFB laser is to change the period of the grating. The period is given by .LAMBDA.=m.lambda..sub.0 /2N.sub.eff where .LAMBDA. is the period of the grating; m is the integer order of the grating; .lambda..sub.0 is the Bragg wavelength (the lasing wavelength); and N.sub.eff, is the effective refractive index of the propagating mode in the active stripe.
A 0.8 nm wavelength separation in 1.55 .mu.m DFB lasers corresponds to a grating pitch difference of 0.13 nm. Such fine control over grating pitch manufacture has been demonstrated by holographic and contact printing techniques. In the holographic technique, resists are exposed using two interfering UV laser beams, and then etched. See, for example, Pakulski et al., "Fused silica masks for printing uniform and phase adjusted gratings for distributed-feedback lasers," Appl. Phys. Lett. 62(3), 222-24; Okuda et al., "Monolithically integrated GaInAsP/InP Distributed Feedback Lasers," Fourth Int'l Conf. Integrated Optics and Optical Fiber Comm., Tokyo, Japan, paper 28B1-4; and U.S. Pat. No. 4,517,280 issued to Okamoto et al. The contact printing technique utilizes an electron beam (e-beam) to directly expose a resist, such as p-methylmethacrylate, that covers the substrate which is to contain the grating. Once the resist is patterned, the grating is etched. See Pakulski, et al.
The holographic technique proves to be rather cumbersome and not readily reproducible in generating grating pitch steps of 0.1 nm or smaller. The contact printing technique requires the reproducible generation of phase masks with grating-pitch steps of 0.1 nm by electron-beam lithography. This is an extremely demanding task for electron-beam lithography which requires extremely accurate and stable beam control down to less than 0.1 nm.
Accordingly, there is a need for a simple, more reproducible technique for varying the lasing wavelength of DFB lasers.